This invention relates generally to magnetic resonance and, more particularly, to methods and systems for generating an image from a multiple field of view gradient coil utilizing a sum and difference approach.
Magnetic resonance imaging systems include gradient coils to generate linear magnetic field gradients which are used for spatial encoding. Gradient coils are typically designed to optimize strength, slewrate, and useful imaging volume. The imaging speed of a gradient coil is roughly proportional to the product of strength and slewrate. Generally, the larger the useful imaging volume, the lower the imaging speed. Hence coils designed for whole body imaging perform less than optimally for lower volume applications, such as, for example, a head application.
Dual field of view gradient coils were introduced in an attempt to overcome this difficulty. In dual field of view gradient coils, a set of two or more electrically separate windings is provided for each gradient axis and a single set of windings can be switched to operate in two or more different modes, whereby one mode provides a higher imaging speed over a small imaging volume, compared to the other mode which provides for a larger imaging volume at a lower imaging speed.
Dual field of view coils, however, do not attain the performance provided by single coils optimized for a given imaging volume, due to the physical space requirements of the primary and shield windings. Imaging efficiency is the imaging speed (strength times slewrate) divided by the current times the voltage provided by the gradient amplifier, for a given imaging volume. The larger the separation between the primary and shield windings, the higher the imaging efficiency.
There have been many attempts at determining the optimal dual field of view coil topology that provides the best imaging efficiency. One approach to optimize the dual field of view coil topology uses single primary and shield formers, and creates two or more switchable circuits on the surface of each former. A problem with this approach is the reduced flexibility with respect to optimizing the field linearity, or alternatively the difficulty in constructing the multiple conductor crossovers.
A second approach uses two separate sets of primary and shield coils. One set of primary and shield coils provides for one imaging volume, and the other set of primary and shield coils provides for another imaging volume. A problem with this approach is that either one set of coils is very inefficient, or both sets are moderately inefficient. This approach is commonly known as the “twin” configuration.
Recently, a “main plus corrector” approach has been developed. In the main plus corrector approach, one coil (the main coil) is used for both imaging volumes, and the other coil (the corrector coil) is used to increase the imaging volume. Hence the small imaging volume is achieved with the main coil alone, and the large imaging volume is achieved with the main coil and the corrector coil operating simultaneously. This approach offers improved imaging efficiency over using two separate sets of coils for each volume. However, in the main plus corrector configuration, the gain (gradient strength per unit current) of both volumes is determined by the gain of the main coil which can be problematic because typically one would prefer to have the smaller imaging volume configuration operate at a higher gain than the larger imaging volume configuration.